1. Field of the Invention
The present invention relates to a tyre for vehicle wheels, in particular for heavy load vehicle wheels.
The invention also relates to a tread band for one such a tyre, in particular a premoulded tread band for covering worn tyres.
2. Description of the Related Art
A tyre for vehicle wheels intended to be used on heavy load vehicles generally comprises at least one carcass structure comprising a structure ply formed by reinforcing cords embedded in an elastomeric matrix. The carcass ply has end edges respectively engaged with annular anchoring structures. The latter are arranged in the zones of the tyre usually identified with the name of “beads” and normally consist each of an annular substantially circumferential insert on which at least one filling insert is applied, in radially outer position thereof. Such annular anchoring structures are commonly identified as “bead cores” and have the task of keeping the tyre well fixed to the anchoring seat specifically provided in the wheel rim, thus preventing, in operation, the radially inner end edge of the tyre coming out from such seat.
At the beads specific reinforcing structures may be provided having the function of improving the torque transmission to the tyre.
In a radially outer position with respect to the carcass ply, a belt structure comprising one or more belt layers is associated, said belt layers being arranged radially one of top of the other and having textile or metal reinforcing cords with crossed orientation and/or substantially parallel to the direction of circumferential extension of the tyre.
In a radially outer position with respect to the belt structure a tread band is applied, made from elastomeric material as well.
On the side surfaces of the carcass structure respective sidewalls of elastomeric material are also applied, each one extending from one of the side edges of the tread band up to the respective annular anchoring structure to the beads.
As is known, the tyres intended to be used on heavy load vehicles are typically required to have optimal traction capability, acceleration, directionality and controllability characteristics, even on snowy or wet road surfaces.
Another characteristic typically required to the tyres is a limited, and/or as much as possible uniform, wear, so as to increase the performances of the tyres in terms of kilometric yield and reduce the running and vibrations on travel.
The aforementioned characteristics in part contrast with each other. In particular, in practice it is not possible to ensure optimal performances in terms of traction capability, acceleration, controllability and directionality without in some manner affecting the tyre performances in terms of kilometric yield and noise/vibrations.
The tyre performances on travel considerably depend on the design of the tread band thereof.
Typically, the tread band of a tyre comprises an annular central portion astride the equatorial surface of the tyre and two annular shoulder portions arranged on axially opposite sides with respect to the aforementioned annular central portion and separated from the latter by respective circumferential groove.
The tread band moreover comprises a plurality of grooves and sipes arranged transversally and circumferentially in order to define a tread design. The aforementioned grooves and sipes define a plurality of blocks in the tread design.
The circumferential and transversal sipes typically have a shape such to define, in the adjacent blocks, respective mutual embedding or constraining portions, both in circumferential direction and in axial direction. The embedding in the circumferential direction contributes to give stability to the tyre, above all in the travel direction, while the embedding in the axial direction contributes to confer stability to the tyre particularly in the lateral direction. Such stability is essentially given by the lesser mobility of the blocks in the ground-contacting area of the tyre due to the mutual abutment between contiguous blocks caused by the closure of the sipes.
Throughout the present description and in the subsequent claims, the term: “block”, is used to indicate a tread band portion delimited by consecutive grooves or sipes both in an axial and circumferential direction, the “groups of block” being instead circumferentially delimited by two subsequent transversal grooves and formed by a number of blocks which are circumferentially aligned and separated by transversal sipes.
Throughout the present description and in subsequent claims, the terms: “grooves” and “sipes”, are used to indicate channels formed in the tread band of the tyre, the transversal and circumferential sipes having a width, respectively in a circumferential and transversal direction, lower than the width in a circumferential and transversal direction of the transversal and circumferential grooves.
Throughout the present description and in the subsequent claims, moreover, the terms: “axial” and “axially”, are used to indicate a direction substantially orthogonal to the equatorial plane of the tyre, i.e. a direction substantially parallel to the rotation axis of the tyre. The terms: “radial” and “radially”, are instead used to indicate a direction substantially orthogonal to the rotation axis of the tyre and lying on a plane passing through such rotation axis, while the terms: “circumferential” and “circumferentially”, are used to indicate a direction substantially parallel to the equatorial plane of the tyre along the annular extension thereof.
Throughout the present description and in the subsequent claims, the expression: “ground-contacting area of the tyre”, is used to indicate the portion of the peripheral surface of the tread band in contact with the road surface.
While the circumferential grooves affect the behaviour of the tyre in terms of lateral stability and directionality, the transversal grooves affect the behaviour of the tyre in terms of traction capability and acceleration. The circumferential and transversal grooves moreover affect the water removal in the ground-contacting area of the tyre while running over wet road surfaces, reducing the aquaplaning phenomena.
A high contribution in terms of traction capability and acceleration is given to the tyre by the grip fronts defined on the tread band by the transversal sipes. Such sipes also contribute, in case of wet road surface, to the water removal.
In the case of snowy road surface, the transversal sipes among other things ensure the trapping of the snow in the ground-contacting area of the tyre, so as to make on travel a contact between tyre and road surface of snow/snow type, such a type of contact being desired as it confers greater traction capability to the tyre. In fact, during the rolling of the tyre, the transversal sipes entering the ground-contacting area of the tyre are closed due to the mobility of the blocks under the ground-contacting area of the tyre, in such a manner trapping the snow, to then be opened by exiting the ground-contacting area of the tyre.
Throughout the present description and subsequent claims, the terms: “entering” and “exiting”, are used to indicate—with reference to the ground-contacting area of the tyre and with regard to the structural characteristics of the blocks of the tread band (or of the transversal sipes)—those portions of the blocks which are first stressed or are the first entering in, contact with the road surface during the rolling of the tyre (or the transversal sipes adjacent to such block portions) and, respectively, the block portions which are last stressed or are the last abandoning the contact with the road surface (or the transversal sipes adjacent to those block portions).
US 2007/0095447 discloses a tread band for tyres provided with groups of block delimited by circumferential and transversal grooves, such groups of blocks being in turn provided with sipes with a variable profile in a radial and axial direction. In particular, the profile of these sipes varies in an axial direction with a sinusoidal progression having a constant pitch, while in radial direction it varies between a radially outer portion of the groups of blocks wherein the sipe has, in a circumferential direction, a predetermined width and a radially inner portion of the groups of blocks wherein the sipe has, always in a circumferential direction, a greater width.
EP 1 669 217 discloses a tread band for winter tyres, comprising a plurality of groups of blocks defined between a plurality of circumferential grooves and a plurality of transversal grooves. Each group of blocks comprises a plurality of transversal sipes having a profile with a substantially zigzag progression both in the axial direction and radial direction.
US 2003/0201048 discloses a tread band for winter tyres, comprising a plurality of blocks defined between transversal grooves, each block comprising a sipe of lower width than that of the transversal grooves and depth substantially equal to that of the transversal grooves. The sipe has a profile with a substantially zigzag progression both in the axial direction and radial direction.